Morpheme knowledge is shaped by information available through orthography.

A large portion of words in a language are formed by combining smaller meaningful units called morphemes (e.g., teach + -er → teacher). Understanding a language's morphology is vital for skilled reading as it allows readers to interpret both familiar and unfamiliar words (e.g., tweeter). It is widely agreed that children rely on reading experience to acquire morpheme knowledge in English, and emerging research suggests that different aspects of this experience may impact affix learning in different ways. We contrasted three potential definitions of what constitutes readers' affix experience using the morpheme interference paradigm with 120 adults. We found that skilled readers' affix knowledge most closely aligns with a definition proposing that affix learning is primarily supported by experience with words in which affixes are identifiable without specialised linguistic knowledge. Due to the nature of morpheme presentation in English orthography, this excludes a significant number of genuinely complex words, while including affix-like patterns in non-meaningful contexts (e.g., -er in corner). This definition also posits that these morphological false alarms actively hinder learning. Our research represents a critical step towards a psychologically realistic theory of morpheme learning from text experience.